I'm creating a campaign that takes players from level 3 to level 15+. The premise is that the party is granted a special role by the reigning good King to protect and serve (essentially they are special ops for the Kingdom). Over time, the players will investigate and protect the nation from threats by going on special missions and such. Hints will be dropped along the way that a major event is going to happen and the party will actively be trying to stop it for the final levels. Ultimately the castle+city they are trying to protect will be attacked and they find that there is no stopping the destruction of the city and castle. They will fight to protect people as they evacuate, but eventually "go down with the ship," so to speak.
The narrative is meant to be crafted much like that of Halo Reach, Rogue One, or similar. Each character will have an important backstory that ties them to the kingdom in a special way. My question is:
Do I tell the players up front their characters are going to die in the end, spoiling the story? Or do I try to craft a narrative where they slowly come to learn their fate?
I fear that if I tell the players their characters will die upfront, it ruins a lot of the suspense and hidden story, but also they may choose to invest less in their characters. On the contrary, if I don't tell them, they might be pissed at the end of the story if somehow I can't bring them down the "self sacrifice lane."
I was reading this related question:
How can I plan a TPK finale that doesn't look planned?. I like some of the answers there, except that the question there is for a mini-campaign on the side. My campaign is meant to take well over a year – and players will hopefully be more invested in their characters.
Best Answer
Surprising your players with this will subvert the expectations the game itself is built upon.
D&D is designed as a fundamentally cooperative story-telling game, and not just in a "the designers intended it to be this way but didn't say it had to be this way", I mean that the idea you've presented here runs against the fabric of the game. We see in the introduction to the Dungeon Master's Guide:
By writing the end of the story before you've gotten there, you have undermined the agency of your players and removed them from the basic formula of the game. You are all supposed to be telling the story together, and each player's choice are supposed to matter. Your plan says to your players that their choices don't matter, and that you only care about telling your own story. This will be a very unpleasant surprise, because this is not the game the players thought they were playing. Your players have come to the table expecting their choices to matter. They expect to have some measure of control of their characters' destinies. And you are planning to deny them these things. You are trying to play a completely different game from the one the players think they are playing.
Tim C has some good thoughts about player agency in this answer, I'll reproduce here the definition and Tim's brief analysis of your idea:
This is a bad plan. Just don't do it. Don't remove the cooperative part of cooperative storytelling. Let your player's decide the fate of their characters. Show them that their choices matter and that you care about what they are bringing to the story; not your story, but your story, and her story, and his story, and their story. You should be working together, but this plan says "No, you are working for me."
To make it work, the players must buy in up front.
The question you linked, How can I plan a TPK finale that doesn't look planned?, has a concise solution for making this work. SSD writes:
What SSD suggests here is exactly how the Dungeon Master's Guide describes the way the game is best played. If you want to do this, you have to pitch it to the players beforehand and get everyone on the same page about how the story will ultimately go down.